The invention relates to a method for monitoring a rotational angle sensor on an electrical machine.
The invention will be described hereinafter with reference to a motor as an example of an electrical machine, but is not limited thereto. The problems with a rotational angle sensor can arise also in machines operated as generators and are solved in corresponding manner.
In order to be able to operate electromotors with a higher dynamic and greater precision, it is often necessary to know the angular position or rotational position of the rotor relative to the stator. This is especially the case with induction machines that are supplied by a converter. In order to obtain that information, a rotational angle sensor is used, which is coupled to the motor shaft. Such a rotational angle sensor is also known by the name xe2x80x9cResolverxe2x80x9d or xe2x80x9cEncoderxe2x80x9d. It may operate in various ways. In some instances it is not the angle of rotation but the angular speed that is measured and integrated. The crucial factor is, however, that it is possible either directly or indirectly, for example by integration, to determine the angular position of the rotor relative to the stator. The information relating to the angle or to the speed is supplied to the frequency converter or to another electronic motor control system and is used to control the speed and/or the torque.
If the rotational angle sensor fails for any reason, the electronic motor control system receives either no signal at all or a false signal. xe2x80x9cFailurexe2x80x9d is to be understood as any circumstance in which the correct rotational angle signal does not reach the motor control system. It may be, for example, an interruption in the signal transmission between the rotational angle sensor and the motor control system. When the motor control system does not receive the correct signal, this results in incorrect behaviour during operation. For example, when customary V-F regulation with rotational speed feedback is being used, the motor control system drives the speed of the motor to its maximum when the signal transmission is interrupted. Clearly such operating conditions are undesirable.
In the prior art, a number of methods are known for detecting or identifying such fault situations in conjunction with the rotational angle sensor, so as to be able subsequently to take measures to correct the fault. For example, redundancy can be incorporated into the rotational angle sensor, that is to say a second rotational angle sensor can be provided and the results of the two rotational angle sensors can be compared with one another. The functioning of the rotational angle sensor can also be monitored by means of integrated monitoring electronics. Another type of monitoring consists of using certain parameters of the motor or of the motor control system in the monitoring process. The present invention is of the latter type.
From U.S. Pat. No. 5,691,611 a method for monitoring a rotational angle sensor is known in which an average speed of the motor is calculated. That speed is compared with the instantaneous value of the speed detected by the rotational angle sensor. An absolute difference is formed from those two values. Should that difference be greater than a limit value, it is assumed that the rotational angle sensor is not functioning correctly.
That method works well at relatively high speeds. The detection of faults becomes critical, however, in the range of very low speeds, for example in the range of from 0 to 100 rev/min. That range also includes the particularly critical situation of a speed of 0 rev/min. When such a speed is indicated, it is necessary to be able to detect whether or not the rotor is actually stationary, for example whether or not it is blocked, or whether or not the rotational angle sensor is defective, for example signal transmission has been broken off or interrupted in some other way.
The problem underlying the invention is to provide a method for monitoring a rotational angle sensor on an electrical machine that enables fault detection over a large speed range.
That problem is solved in a method of the type mentioned at the beginning in that the electrical power of the machine is measured and a power value is estimated using the output signal of the rotational angle sensor, a residual being formed from the measured power and the estimated power and the time curve of the residual being monitored.
By proceeding in this way it is possible to carry out fault detection also at low speeds and even at extremely low speeds. A blockage of the rotor is also detected, that is to say fault detection can be carried out even at a speed of 0. The fault detection is, moreover, substantially faster. The same method can be used over the entire speed range. This method also enables a relatively good ratio of signal to interference (signal/noise ratio) to be obtained. The method is also relatively robust because it is dependent upon parameters in the motor model that exhibit little sensitivity to influences causing disturbance. In the simplest case, the residual is formed from the difference between the measured power and the estimated power. When the time curves of the measured power and of the estimated power vary, the time curve of the difference is also not constant. That difference, hereinafter referred to as the residual, can be used to check the functioning of the rotational angle sensor. The electrical power can be measured with relatively little complication, generally from the product of the current and voltage. The estimated power is calculated in part from values that are determined by measurement, for example currents or indeed the speed, and also from parameters of the machine, for example inductance or the like. Where there are no faults, the two powers, that is to say the measured power and the estimated or calculated power, should correspond to one another. Should the rotational angle sensor or its output transmission be disrupted, the part of the estimated power based on that signal, that is to say on the speed or the angle of rotation, will necessarily change. This results in a difference, or a residual, between the measured power and the estimated power, indicating a fault. That procedure also functions when the rotor is stationary because, for example, it is blocked. In that case the electrical power consumed by the machine can be measured. That power is, however, pure power loss because no mechanical work is done. In an appropriately selected machine model, the same value will result for the estimated power also. Since the speed signal is zero, here again it should only be a question of power loss components. The situation is different when the rotational angle sensor is defective and indicates a speed of zero even though the rotor is in fact rotating. In that case, the measured electrical power is higher by the amount produced mechanically at the rotor shaft. When the rotor is blocked, in many cases it is not completely stationary but will rotate a little bit. Even that small amount of rotation is enough to be able to check whether or not the rotational angle sensor is functioning, since both the measured power and the estimated power increase to the same extent. The result is that when there are no faults the residual remains zero.
Preferably an error threshold is specified and it is assumed that there is an error in the output signal when the residual exceeds the error threshold. This is a quasi-static procedure. Relatively small differences between the measured power and the estimated power cannot be avoided during operation, especially when there are changes in speed. These are not critical, however, provided they do not exceed a certain value. If such a value is exceeded, it is assumed that there is a fault.
In an alternative or additional embodiment, it can be assumed that there is a fault when the increase in the residual over time exceeds a predetermined limit value. When the change in the difference between the measured power and the estimated power becomes too large, it is generally a clear indication that a fault is present.
Preferably the estimated power is discarded within a predetermined period after the error threshold has been exceeded. It can be observed, for example, that the curve of the residual in some cases initially rises and once a maximum has been reached falls again, namely below the zero value. This is caused, inter alia, by the fact that, for example in a control system based on the feedback of the rotational angle, the current is calculated incorrectly. The incorrectly calculated current is also used, however, in the calculation of the estimated power, with the result that a still greater error can occur there. In order to prevent a fault-free state from being detected by mistake when the zero line is crossed, the estimated power is discarded within a predetermined period of time. The machine will preferably still be halted or switched to a different controlling or regulating procedure that no longer requires the feedback of the angle signal.
Accordingly, when a fault occurs, the output signal of the rotational angle sensor is preferably no longer used to control the machine, so avoiding further errors.
Preferably the estimated power is formed by a load part, a rotor loss part, a stator loss part and a magnetizing part. That model reproduces the electrical power consumption with sufficient accuracy and can accordingly be used to form the residual.
In that instance, it is especially preferable that the output signal of the rotational angle sensor enter only into the load part. This facilitates the calculation. A clear separation can be made between the signals. The calculation is also simplified.
Preferably the machine is shut down gently after a fault occurs. In other words, the machine is not braked abruptly, which could result in further damage, but is run down slowly. Alternatively, it is possible to switch to a safety operating mode in which the power capacity is reduced.